| Borated stainless steel(BSS)is the important neutron absorber material and is widely used in the field of nuclear power fuel/spent fuel transportation and storage.Improving the mechanical properties and boron content of the BSS,and integrated forming are important directions in the research of BSS for neutron absorber materials in recent years.In this study,BSSs were prepared by powder metallurgy process incorporating atomization and hot isostatic pressing(HIP)sintering.The microstructure evolution,distribution of phases and elements,mechanical properties and heat deformation stress-strain curves of alloys after the HIP,heat treatment and heat deformation were studied and analyzed by using transmission electron microscopy(TEM),scanning electron microscopy(SEM),Gleebe-3500 and other test methods.By summarizing and analyzing the results,the growth kinetic equations were established;the effects of process conditions and microstructure on the properties were analyzed.Firstly,the HIP processes of three BSSs with 0.29wt.%,1.86wt.%and 3.29wt.%boron content and high borated steel with 1.5wt.%boron content were studied.The phases of BSSs were austenite and Fe1.1Cr0.9B0.9,and the phases of high borated steel were lamellar pearlite(soxhite)and Fe2B.Through the analysis and summary of microstructure,element distribution,relative density and physical phase of boron content 1.86wt.%BSS at different HIP temperatures(600℃～1160℃),the mechanisms of boron containing phase precipitation,nucleation location,elemental diffusion driving force and growth kinetic equations were obtained.The diffusion of chromium was found to be the controlling factor for the growth of the second phase.The mechanical properties of the specimens,which were BSS containing 1.86wt.%boron by HIP at different temperatures,were tested:The highest tensile strength was 783 MPa and the highest elongation was 19%.The BSS with 3.29wt.%boron content prepared by HIP,which had the uniform microstructure and elements distribution,was one percentage point higher than the maximum boron content(2.25wt.%)specified in ASTM A887.The boron-10(10B)surface density of the BSS with the 1.86wt.%boron content was measured by chemical and neutron absorption methods and the 10B surface density of the 3 mm thick plate was 7.59 mg/cm2.Three grades of BSS with 0.29wt.%,1.86wt.%and 3.29wt.%boron content were operated to solid solution treatment from 900℃ to 1200℃.The microstructures of the quenched alloys were analyzed.It was found that as the solid solution temperature increased,the solubility of boron in austenite increased and the boron distribution became more uniform.The higher the temperature,the larger and more regular the grain size of the second phase.Especially for the BSS with the 3.29wt.%boron content,the particle size of the second phase was larger,more regular in shape and with greater particle distance as the solid solution temperature increased.With the increase of solid solution temperature,the tensile strength of 1.86wt.%BSS showed the small decrease from 804 MPa to 745 MPa;the elongation showed the increasing trend from 1 8%to 23%.Hot compression tests were carried out using Gleebe-3500 on BSS with the 1.86wt.%boron content at temperatures ranging from 900℃ to 1100℃ and strain rates of 0.01 s-1～10 s-1.The true stress-strain curves were obtained and the activation energy(Q)was calculated to be 442.35 kJ/mol.The critical strains for dynamic recrystallization of the BSS were 0.022～0.122,lower than boron-free AISI 304 stainless steel;the critical stresses were 146.69 MPa～386.53 MPa,higher than boron-free AISI 304 stainless steel.BSS plate of 3900 mm long,240 mm wide and 3 mm thick was prepared by hot rolling.The BSS developed by this study has been used in special equipment of Precision Machinery&Engineering Research Co.The BSSs of this study have been supplied to a number of companies such as Orano and Skoda for trials. |
PDF Full Text Request